Heretofore, attempts have been made to reduce the response motions of a large floating body in the sea by adopting a form that couples a large mass of seawater exterior to the body so that its heaving amplitude is damped, as disclosed, for example, in U.S. Pat. No. 4,115,343 to Finsterwalder.
A cylindric body of upright axis form having an outer cylindric wall extensively perforated by transverse flow-guiding channels and having an annular chamber within the wall, with a highly apertured chamber floor, has been proposed to dissipate energy of water motion in a zone exterior to the body, for reducing wave amplitude and heave, as taught in U.S. Pat. No. 3,299,846 to G. E. Jarlan.
Other approaches to the problem have relied on locating the major buoyant volume of the body at sufficient depth to reduce heave while supporting a superstructure above the sea by slender columns, as in known semi-submersible platforms.
Still other forms dispose an air-filled chamber below mean sea level and opening downwardly so that heave compresses the air and reduces buoyancy as a function of the heaving force, as in U.S. Pat. No. 4,241,685 to G. L. Mougin.
In all prior forms so far devised it has not yet proved possible to achieve the very great load-carrying capability and the high stability desired for stationary floating structures intended, for example, to support a platform for carrying out drilling for petroleum under the seabed, and for producing and servicing a large cluster of oil and gas wells. Still greater load capacity would be required if the body could also accomodate process plants. The need to supplant semi-submersible platforms which have been relied on for supporting equipment is evident in the number of disastrous failures resulting from sea states.
A floating platform-carrying body in the open sea will be exposed to long-period, high-amplitude ocean waves and wave groups of periods 12 to 22 seconds or longer. Wave heights of such longer-period waves when at their partially breaking states may range from about 19 meters to 33 meters or more as measured from trough to crest. It will be obvious that when tethered cylindric bodies having bluff sidewalls of large area are impinged by gravity waves the relative motions of the surrounding volume of seawater will exert forces on the body. These forces arise from the flow velocities and comprise drag and inertial forces, the inertial force considerably exceeding in magnitude the drag force. A virtual mass of seawater around the obstacle is involved in the relative motion such mass being defined as that volume of fluid which experiences acceleration because of the presence of the obstacle. The virtual mass increases with the degree of wave reflection by the obstacle, and hence is a function of the form and surface porosity thereof.
The response of the obstacle to the forces will be translational accelerations, causing it to be moved through the surrounding fluid. Such motion in turn brings about a retarding drag and an opposing thrust due to inertial reaction by the invaded water mass. The relationship between the resultant force and the acceleration may be represented, for example for horizontal motion of the obstacle, simply as F=M.a, where the mass M is the tensor sum of a virtual inertial mass and the mass of the obstacle itself.
A body having unperforated bluff walls, loosely tethered, will sustain periodic motion, tracing large closed loops as it experiences motions pertaining to a system having three degrees of freedom, namely heave (vertical displacement), surge (horizontal sliding motion), and rolling or pitching (rotation about a horizontal axis). Floating support structures known in the prior art having shallow or moderate drafts, and diameters under 100 meters, even though provided with perforated shell walls and a partially perforated bottom, will have poor stability to large waves. If such support structure is enlarged for load-carrying capabilities adequate for well drilling, for example in sizes up 200 meters diameter, their responses to the longer-period waves would render drilling work dangerous.
Ideally, the platform from which deep sea drilling work is carried out should have a vertical displacement under wave conditions averaging about 15 meters wave height, of well under 4 meters, and pitching rotations below .+-.4.degree..
The problem of achieving reduced heave response cannot be separated in a tethered floating body from the problems of suppressing surging, rolling and pitching motions, particularly in view of the need to minimize wave reflection so that horizontal drag and inertial forces do not impose excessive loads on the anchoring system.